Method and apparatus for selective corona treatment of toner particles



May 13, 1969 P. J. MALINAR IC METHOD AND APPARATUS FOR SELECTIVE CORONA TREATMENT OF TONER PARTICLES Sheet Filed Oct. 11, 1966 INVENTOR. PAUL J. MALINARIQ Q m M St A TTORNEVS y 13, 1969 P. J. MALJNARIC 3,444,369

METHOD AND APPARATUS FOR SELECTIVE CORONA v TREATMENT OF TONER PARTICLES Filed Oct. 11, 1966 Sheet 2 of 2 INVENTOR. PAUL J. MALI NARIC BY mjg Q ATTORNEYS United States Patent M 3,444,369 METHOD AND APPARATUS FOR SELECTIVE CORONA TREATMENT OF TONER PARTICLES Paul J. Malinaric, Penfield, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Oct. 11, 1966, Ser. No. 585,825 Int. Cl. G01n 21/34 US. Cl. 250-65 9 Claims ABSTRACT OF THE DISCLOSURE Method and apparatus for the reduction of background in transferred xerographic copy. A developed toner image on a photoconductive surface is subjected to a low level corona discharge of a polarity opposite the charge on the toner particles overlying the image areas. The corona discharge adjacent the image areas will be repelled by the like but higher charge pattern in the image areas of the photoconductive surface to thereby render image area toner uneifected. The corona discharge adjacent the non-image areas of the photoconductive surface will not be repelled and will thus convert the charge on the toner overlying the non-image areas to a polarity opposite that on the image area toner particles. This will permit the electrostatic transfer of either the image area toner or the nonimage area toner to a backing sheet.

This invention relates in general to xerography and in particular to a method and apparatus for preventing the transfer of toner powder to non-image areas of Xerographic copy.

In practice of Xerography, as described in US. Patent No. 2,297,691, to Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic images. In the usual method of carrying out the process, the xerographic plate is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original light pattern.

The latent electrostatic image can then be developed by contacting it with a finely divided electrostatically attractable material such as a resinous powder. The powder is held in image areas by the electrostatic charges on the layer. Where the charge is greatest, the greatest amount of material is deposited; and where the charge is least, little or no material is deposited. Thus, a powder image is produced in conformity with the light image of the copy being reproduced. The powder is subsequently transferred to a sheet of paper or other surface and suitably aflixed to thereby form a permanent print.

The electrostatically attractable developing material commonly used in xerography consists of a pigmented resinous powder referred to herein as toner. In order for the toner to attach itself to the electrostatic charge corresponding to the image areas, it is first necessary to charge the toner particles to a polarity opposite from the charge on the image areas.

In the most common form of development, generally called cascade, the toner is mixed with a coarse granular material called carrier. The carrier is usually a glass or sand bead coated with a material removed in the triboelectric series from the toner so that a triboelectric charge is generated between the powder and the granular carrier upon mutual interaction. Such charge causes the toner powder to adhere to the carrier. When the toner ladened carrier is moved into contact with a latent elec- 3,444,369 Patented May 13, 1969 trostatic image, the charge of the image areas attracts the toner from the carrier to the image areas to thus render the latent electrostatic image visible.

In the practice of xerography, toner powder often becomes deposited on portions of the xerographic surface corresponding to non-image areas. The principal causes of such unintended deposition are uncontrolled toner clouds and incomplete scavenging.

Uncontrolled toner clouds occur when toner ladened carrier granules are dropped and cascaded across an image bearing photoconductive surface. The forces of impact between the developer and photoconductive surface jar appreciable quantities of toner powder free from their associated carrier granules. This toner powder then assumes a cloud-like mass which may settle on all areas of the photoconductive surface including non-image areas, obviously an undesirable result. This feature of cascade development is aggravated when a high toner concentration is used, that is, when excessive amounts of toner particles have been mixed with a lesser quantity of carrier granules.

Scavenging occurs when carrier granules move across a toner bearing surface after depositing their previously associated toner powder onto image areas. As the granules continue their movement across the surface, they contact and attract those toner particles which are held to the photoconductive surface by weak electrostatic forces.

Weak electrostatic forces are characteristic of attractive forces holding toner powder to non-image areas. Consequently, scavenging carrier granules tend to remove toner powder which has been undesirably deposited in non-image areas. Incomplete scavenging occurs when insufficient toner-free carrier granules are moved across the image bearing surface. As above, this background producing effect may be aggravated by over-toned carrier particles. This is because over-toned cascading carrier granules take longer to reach the point where they have been sufficiently depleted of toner so that they may begin to scavenge.

In addition to the above described causes of toner deposition on non-image areas, such deposition may also be caused by toner particles becoming charged to a polarity opposite from that intended. When this happens these particles are repelled from image areas to non-image areas. Toner powder may also assume a neutral polarity in which case it may also settle on non-image areas.

The present invention is directed to reducing the deleterious effects of toner particles deposited in non-image areas of a developed image, regardless of their polarity, the manner by which they were deposited or the method of development employed.

According to the present invention, the developed image-bearing photoconductive surface is exposed to a low corona current immediately after development. By proper selection of the polarity and magnitude of this background clean-up corona current, the toner powder in non-image areas can all be charged to a polarity opposite from that of the toner particles corresponding to the image areas. This is achieveable with no degrading of the charges on the image area toner.

The resulting polarity difference between the toner powder in the image and non-image areas, makes possible, with the aid of a transfer corotron, the transfer of only image area toner and, consequently, a background free area toner particles from the developed photoconductive surface to thereby leave a background free image on the photoconductive surface. The first alternative is readily adaptable for incorporation into existing xerographic machines adapted for continous and automatic operation. The second alternative is of special interest when considering xeroprinting, xerographic master making and lithography. These applications of the present invention should be considered as illustrative only, inasmuch as the basic concept of this invention is applicable throughout the field of xeropraphy.

It is a further object of the present invention to eliminate unwanted background in xerographic copy and masters.

A further object of the present invention is to eliminate the transferral of toner powder from non-image areas of xerographic copy.

A further object of this invention is to effectuate the charging of all non-image area toner powder to a polarity opposite from all image area toner powder in a developed xerographic surface to thereby allow the transfer of only like-polarity toner powder from the developed imagebearing surface.

A further object of the invention is to remove unwanted background toner powder from xeroprinting masters.

These and other objects of the present invention are achieved by moving developed xeropgaphic surfaces relative to a low current corona of a polarity opposite from the polarity of toner powder deposited in image areas of the developed xerographic surface. This procedure will then charge non-image area toner to a polarity opposite from the toner in image areas to thereby permit the transfer of only the image area toner, or in the alternative, the transfer of only non-image area toner for the ultimate production of background-free copy.

For a better understanding of the invention, as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in conjunction with the accompanying drawings wherein:

FIGS. 1-3 are diagrammatic front cross sectional views of a xerographic surface being processed in accordance with the invention;

FIGS. 4-7 are diagrammatic front cross sectional views of a xerographic surface being processed in accordance with a second embodiment of the invention; and

FIG. 8 is a diagrammatic side cross sectional view of a xerographic machine adapted for continuous and automatic operation adapted to carry out the process of the instant invention.

The principle upon which this invention is founded is that when a corona emitting device, such as a corotron described for example in US. Patent No. 2,836,725, is positioned over a photoconductive surface, it will tend to charge the photoconductive surface therebeneath up to an equilibrium voltage with the corotron. The equilibrium voltage for any system is determined by such variables as the corotron diameter, the medium and distance between the corotron and photoconductive surface as well as the potential difference therebetween. The equilibrium voltage must fall between these potentials. When the photoconductive surface, or any part thereof, is of a lesser than equilibrium voltage, then the photoconductive surface will accept the corona emissions. By accepting the emissions, the corotron will thus raise those portions of the surface with a lesser than equilibrium voltage up to the equilibrium voltage. If, however, the equilibrium voltage equals or is lower than the voltage already on the photoconductive surface beneath the corotron, no charge, can be accepted by the surface and the surface in such areas will remain electrically unchanged. This is because the potential difference between the photoconductive surface and corotron is insufficient to support a flow of emissions to the photoconductive surface. Emissions continue to flow from the corotron under such conditions but are repelled by the higher like-potential fields adjacent the photoconductive surface and are directed to the corotron shield. If no shield were employed, the corona emitting wire would raise the surface up to the equilibrium voltage and then cease emitting so long as the potential difference remained less than that required to produce emissions.

This tendency of a photoconductive surface adjacent a corotron to reach an equilibrium voltage is a one way process. The emitted corona will have no effect on photoconductive surface portions with potentials equal to or greater than the equilibrium voltage. It will, however, tend to raise the lesser than equilibrium voltage portions of the photoconductive surface up to the equilibrium voltage and then cease having an effect on those areas. The corona ceases to have an effect on such areas since they are no longer below the equilibrium voltage. In this sense, the equilibrium voltage may be considered a corona cut off voltage. It has also been determined that the portions of the photoconductive surface wherein the voltage beneath the equilibrium voltage is farthest therefrom, Will start becoming charged first and fastest.

The manner in which this principle is advantageously applied to xerography is readily illustrated by reference to the drawings. Turning to FIGS. 1-3, the xerographic surface 10 is composed of a conductive substrate 12, coated with a photoconductive layer 14, selenium for example, upon which latent electrostatic images may be formed. The photoconductive surface may be of the conventional types usable in the field of Xerography capable of retaining latent electrostatic images thereon.

FIG. 1 represents schematically, a cross sectional view of a photoconductive surface after development. The positive signs within the photoconductive layer represent undischarged portion 16 of the image photoconductive surface corresponding to image areas of the copy being reproduced. The portions of the photoconductive layer without such positive signs represent discharged portion 18 of the imaged photoconductive surface which correspond to non-image areas of the copy being reproduced. The negative marked circles adjacent the image areas represent negative toner particles desirably held in image areas by the electrostatic attraction between the dissimilar polarities and are properly oriented on the photoconductive surface for image reproduction. Also located on the photoconductive surface are a variety of toner particles of positive negative and neutral (0) polarity located in positions representing non-image areas. If these particles were to be transferred and fused to a final backing material they would manifest themselves as unwanted background.

In the past it has been the practice in xerography to transfer this background-carrying image to a final copy for fusing. In accordance with the instant invention, however, the developed image is first subjected to a background clean-up step. This is accomplished by merely moving the corona-emitting corotron 20 relative to the photoconductive surface, in close proximity thereto, as shown in FIG. 2. The corotron must be at a very low current and of the opposite polarity as the toner overlying the image areas of the photoconductive surface. This would be positive in the illustrated embodiment.

As the corotron and photoconductive surface move relative to each other, the emitted corona will begin to raise those portions of the photoconductive surface representing non-image areas up to the equilibrium potential. Due to the fact that toner particles of varying charge and consequently varying potential, both positive and negative, overlie these non-image areas, the sprayed nonimage areas will become charged in accordance with the emission charge concurrently with their overlying toner particles. A potential opposite from that found in the toner of the image areas can thus be imparted to all nonimage area toner regardless of their original charge.

The toner particles overlying the image areas are unaffected by the presence of the corona emission. This is due to corona suppression. By corona suppression it is meant that positive emissions from the corotron will be repelled from the photoconductive surface by the higher like-potential field representing the image areas. Consequently, the corona has no effect on image areas and their overlying toner particles. This repelling action is most pronounced over extended image areas. This suppression of corona is illustrated by the absence of emission lines over the extended image area of FIG. 2. In smaller image areas, corona suppression exhibits itself as a bending of the corona emissions around the highly positive image areas to thereby reach adjacent non-image areas. Note the bent emission lines around the small image area and at the edges of the extended image area of FIG. 2. This bending of the corona emission around small image areas is sometimes referred to as the island effect. In either event, Whether the corona emissions are repelled by extended image areas or merely bent around smaller image areas, the same result of no charge variation in image areas occurs with no effect to the image area toner.

While it might be thought that the presence of image area toner of a first polarity will neutralize the image area charge of an opposite polarity to thus negate the effect of the clean up corona, such has been found to be of no problem. It is determinable empirically that in practice, the charge of the toner is generally less than 20 percent of the charge of the photoconductor in image areas. As such, the great majority of the image area charge is still effective even after development. This leaves adequate charge in image areas to permit the corona suppression needed to practice this invention.

The polarities of the toner particles after being subiected to the corona emissions can be seen in FIG. 2. Note these toner polarities as compared with FIG. 1. This background clean up step is usually done in the dark according to conventional practices so as the charge of the image areas of the photoconductive surface are not dissipated by light striking these areas.

After rendering the image area and non-image area toner particles of dissimilar polarities by the corona emission spray, the image area toner may then be transferred to the final backing sheet 24 in the conventional manner for subsequent fusing. The use of a positive transfer corotron 22, as shown in FIG. 3, will facilitate transfer of only the image area toner to the backing sheet. The non im-age area toner, having been charged to a polarity opposite from the image area toner, will be repelled from the backing sheet due to its like polarity to the transfer corotron.

While this embodiment has been made With respect to transferring the image to a backing sheet, it should be understood that the transfer may be made to any backing surface including paper, cloth, etc., as well as to an intermediate for master-making.

The use of the positive image area charge, positive clean-up corotron and positive transfer corotron with negative toner particles has been selected for merely illustrative purposes. As will be understood by anyone skilled in the xerographic arts, development with positive toner and negative image area-s or even reversal development, i.e., back-ground development by like polarity toner and image areas, can be carried out in the practice of the instant invention by proper selection of the various polarities. In reversal development Where, for example, a negative optical image is used to dissipate an original negative charge, on an appropriate xerographic surface, the resulting charge configuration produces a latent electrostatic image with the undissipated negative charges representing background areas. To produce a positive reproduction from this latent image, negatively charged toner would be employed with a positive clean up corotron. In light of this example, the instant invention should be considered in its broadest sense as being directed to a clean up corotron polarity opposite from that of the toner overlying those areas desired to be developed. The clean up corotron polarity need not always be the same as the original surface charging polarity.

The invention herein is applicable independent of the development method employed. This is because the toner polarity reversal is basically a function of the photoconductor charge distribution. The other element of the system, beyond the proper polarity used, merely involves the retention of the original photoconductor charge distribution up to the time of the low level clean up charging.

The second embodiment of the invention as illustrated in FIGURES 4-7 relates to an embodiment quite similar to the first mentioned embodiment but difiering therefrom in the transfer procedure. FIGURES 4 and 5 of this embodiment are identical with FIGURES 1 and 2 of the first embodiment. In this respect the two embodiments are identical in the development and precleaning steps, that is, the resulting developed surfaces with dissimilar toner polarities in the image and non- .image areas are the same. According to the second embodiment, however, a preliminary transfer corotron 26 is employed with a polarity opposite from the conventional transfer corotron 22 of the first embodiment. Note FIG. 6. This preliminary transfer corotron would thus be of the same polarity as the image area toner charge. When a preliminary backing sheet 28 is placed into contact with the developed photoconductive surface with the reverse polarity preliminary cleanup corotron 2 6 behind the preliminary backing sheet 28, all of the non-image area toner will be removed from the xerographic surface and held by the preliminary backing sheet 28. This will leave the background representing areas of the xerographic surface free from toner for subsequent transfer of in the conventional manner as shown in FIG. 7. As described with respect to FIGURE 3, this subsequent transfer includes a conventional backing sheet -30 and transfer corotron 3-2. In the alternative, it may be desired to leave this background free toner image on the xerographic surface for xeroprinting or the like.

This particular embodiment of the invention is especially adapted for xeroprinting wherein a toner image is formed and fused onto a conductive substrate coated with a photoconductive layer. This process will produce a xeroprinting master. It is also suited for fiat plate operations as is the first embodiment. Furthermore, this embodiment could be employed in continuous and automatic xerographic machinery but would require the use of a preliminary clean-up backing sheet to be fed through the machine under the influence of a reverse polarity preliminary transfer electrode prior to the conventional transfer. This preliminary sheet could take the form of an endless web positioned between the clean-up electrode and conventional transfer station. The first embodiment would not require such a preliminary clean-up step prior to transfer. In this respect, the first embodiment would be more suitable for incorporation into continuous and automatic xerographic machinery of the present commercial variety.

The preliminary transfer paper or background clean-up web may be of any dielectric material or conductorbacked dielectric. A conductive material contacting the photoconductive surface would short out the fields on the photoconductive layer and thus remove the change orientations previously placed on the photoconductive surface and the toner. While paper is a suitable dielectric for removing the non-image area toner, it has been found that smooth surfaces such as Mylar or Teflon achieve a more complete removal of this background toner. The smooth surface for toner removal is desirable since it presents a surface which more completely covers and contacts all areas of the photoconductive surface. A nonimage area toner particle will not readily transfer to a rough surface if complete contact is not made therebetween. From another aspect, a rough preliminary transfer sheet is undesirable since it is likely to abrade and remove toner depositions from image areas. In either event, the smooth surface is preferred.

In the embodiment of FIG. 8, there is illustrated a machine constructed for continuous and automatic operation. All of the processing stations referred to by letters are conventional in the xerographic art. For the purpose of the present disclosure, the several xerographic processing stations in the path of movement of the xerographic surface may be described as follows:

A charging station A, at which a uniform electrostatic charge is deposited on the photoconductive layer of the xerographic drum;

An exposure station B, at which the light or radiation pattern of copy to be reproduced is projected onto the drum surface to dissipate the charge in the exposed areas thereof to thereby form a latent electrostatic image of the copy to be reproduced;

A development station C at which a xerographic developing material including toner powder having an electrostatic charge opposite to that of the latent electrostatic image, are moved into contact with the drum surface, whereby the toner particles adhere to the latent electrostatic image to form a xerographic powdered image in the configuration of the copy being reproduced;

A transfer station D, at which the xerographic powder image is electrostatically transferred from the drum surface to a transfer material or a support surface;

A drum cleaning and discharging station E, at which the drum surface is brushed to remove residual toner particles remaining thereon after image transfer, 'and at which the drum surface is exposed to a relatively bright light source to effect substantially complete discharge of any residual electrostatic charge remaining thereon.

As stated above, all of these stations are conventional in the xerographic arts. Situated, however, between the development station C and tarnsfer station D is the background clean-up corotron device which constitutes the basis of the instant invention.

The corotron device 34 operates in the same manner as that described with respect to FIGURES 1-3 and 4-7. As the developed image passes beyond the development station, the developed image passes beneath the clean up corotron 34. The corotron must have the opposite polarity as the charge on the toner on image areas. The corona emissions being of a low current will then render all of the non-image toner particles to a polarity opposite from all of the image area toners. The developed image will then pass through the transfer station for transferring only image area toner particles to a backing sheet for subsequent fusing. The transfer is made under the influence of a conventional transfer corotron 36. The drum will then continue its rotation past the cleanup station for the removal of all of the untransferred toner particles including all non-image area toner. This portion of the photoconductive surface of the drum is now ready for subsequent charging, imaging and development in a continuing cycle for operation.

While it has been explained that the use of the background cleanup corotron is effective in eliminating background in the developed image and final copy, this ability has another beneficial aspect. When the photoconductive surface becomes extremely weak in its ability to hold image-representing charges and toner, as may occur through prolonged use, or, as may also happen, when the toner concentration is substantially reduced, toner deposition in image areas becomes very light. When this happens in conjunction with relatively high background, it becomes diflicult to distinguish between image areas and non-image areas in the final copy. This is most pronounced in line copy as in typing. Consequently, for example, the center portion of the letter 0 may be tonerwise indistinguishable from the letter itself and the letter will appear as an entire dark spot. Furthermore, if two letter ls reside side by side, a heavy toner background between these two letters may be indistinguishable from the light letters themselves and the two letters may appear as a solid box. This fault is often referred to as intercharacter toner deposition. When, however, the clean up corotron of the instant invention is employed to remove this intercharacter toner, very light deposits of image area toner become distinguishable due to the absence of background. Consequently, the use of a cleanup corotron in xerographic machines will tend to improve the readability of copy even when the toner deposition in image areas is comparable to the toner deposition in background areas.

While the instant invention is operable through a wide range of conditions, one set of illustrative parameters in which the invention is operable includes an original photoconductive plate charging of 800 volts with an imaging to bring image areas down to 650 volts and non-image areas down to 40 volts. A cleanup corotron, /2 inch from the drum, may be employed at 0.20 microamps per inch. This would be approximately two microamps when a 10 inch corotron is used. It should be understood that these operating conditions are by way of example only since wide ranges of operating conditions may be employed.

While the present invention as to its objects and ad vantages, has been described herein as carried out in specific embodiments thereof, it is not desired to be limited thereby; but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. In xerographic processing apparatus of the type having a charging station to deposit an electrostatic charge on a photoconductive surface, an exposure station adapted to dissipate the charge in a patterned configuration of image and non-image areas corresponding to the image to be reproduced, a development station at which a xerographic developing material is adapted to be moved into contact with the photoconductive surface to thereby deposit charged toner powder on the photoconductive surface in a configuration corresponding to the image to be reproduced and a transfer station adapted to transfer toner adhering to the photoconductive surface to a backing material, the improvement comprising,

means positioned between the development station and the transfer station adapted to subject the developed Xerographic surface to a low level corona discharge having a polarity opposite from the charge on the toner overlying the image areas to thereby convert the toner particles overlying the non-image areas to a polarity opposite from the polarity of the toner particles overlying the image areas for facilitating the selective transfer of toner particles in one of said areas toner, from the photoconductive surface.

2. The apparatus as set forth in claim 1 wherein the transfer station includes electrostatic means biased to facilitate the electrostatic transfer of the image area toner from the xerographic surface to a backing material.

3. The apparatus as set forth in claim 1 wherein the transfer station includes electrostatic means biased to facilitate the electrostatic transfer of the non-image area toner from the xerographic surface to a backing material.

4. The apparatus as set forth in claim 3 and further including supplemental means to facilitate the electrostatic transfer of the image area toner from the xerographic surface to a final backing material after the removal of the non-image area toner therefrom.

5. In the method of producing powdered xerographic images which includes the steps of charging a photoconductive surface with an electrostatic charge, exposing the charged photoconductive surface to discharge portions of the charge in a configuration of image and non-image areas corresponding to the copy to be reproduced, developing the photoconductive surface with charged toner particles and transferring toner particles from the photconductive surface to a backing material, the improvement comprising,

subjecting the developed photoconductive surface to a low level corona discharge having a polarity oppo site from the charge on the toner particles overlying the image areas, between the developing and transferring steps, to thereby render the toner particles overlying the non-image areas of a polarity opposite from the toner particles overlying the image areas for facilitating the transfer of toner particles in one of said area, from the photoconductive surface.

6. The method of producing powdered xerographic images as set forth in claim wherein the transferring of toner particles from the photoconductive surface to a backing material is done by electrostatic means biased to a polarity opposite the charge on the image area toner.

7. The method of producing powdered Xerographic images as set forth in claim 5 wherein the transferring of the toner particles from the photoconductive surface to a backing material is done by electrostatic means biased to a polarity opposite the charge on the non-image area toner.

8. The method of producing powdered xerographic images as set forth in claim 7 and further including transferring the image area toner from the xerographic surface to a final backing material with the aid of electrostatic means after the transfer of the nonimage area toner.

9. In the method of producing powdered xerographic images which includes the steps of charging a photoconductive surface with an electrostatic charge, exposing the charged photoconductive surface to discharge portions of the charge in a configuration of image and non-image areas corresponding to the copy to be reproduced, developing the photoconductive surface with charged toner particles, the improvement comprising,

subjecting the developed photoconductive surface to corona emissions having a polarity opposite that on image area toner particles and sufficient in magnitude to convert the toner overlying the non-image areas to a polarity opposite that on the toner overlying the image areas, the corona emissions being of insufiicient magnitude to change the polarity of toner overlying the image areas and electrostatically transferring like polarity toner from the photoconductive surface.

References Cited FOREIGN PATENTS 222,058 7/1958 Australia.

RALPH G. NILSON, Primary Examiner.

S. C. SHEAR, Assistant Examiner.

U.S. Cl. X.R. 

